The invention relates to a method and a device for measuring the flow area at the point of maximal restriction in a gas flow passage, in particular for measuring the vane or nozzle area of a gas turbine engine.
Stators, also known as vane rings, are an array of stationary airfoils that are used to change the direction of an annular airflow as it approaches or departs from an array of rotating blades on a turbine or compressor rotor for example. In order to change the minimum flow area through a stator vane ring, adjustments to the trailing edge angle of stator vane blades are made. The minimum flow area is determined by the distance between a vane trailing edge and the next vane's pressure side, and so changing the trailing edge angle changes the minimum flow area. The minimum flow area controls the pressure ratio of the turbine and the mass flow of the engine, and therefore the compressor's running line.
In a new engine the process of tuning the stator to the rotor is relatively simple since the rotating blades are exactly the same. However, as the engine wears and is overhauled, the stator airfoils must be individually adjusted to retune the stator to achieve optimal engine performance. These adjustments may involve simply bending the trailing edge of a stator airfoil, cutting back the trailing edge or in the case of a segmented vane ring, vane segment replacement. However, generally hundreds of minute bends or adjustments must be performed around the stator ring, which accumulate to affect the flow area of the stator.
To calibrate the stator ring relative to the gas turbine engine, the flow area of the stator must be determined. A change in the stator flow area changes the compressor running line, which effects gas generator speed, compressor pressure ratio, temperature, mass flow at constant power and engine surge margin in a transient regime. At constant power, increasing the power turbine stator flow area while maintaining a constant compressor turbine stator flow area increases gas generator speed and mass flow but decreases the compressor pressure ratio slightly. Therefore, vane matching based on effective flow area is a critical engine overhaul procedure for predicting optimum engine performance and achieving optimum efficiency and energy consumption.
Conventionally, the flow area of a stator ring can been determined by use of a sonic flow rig as for example shown in U.S. Pat. No. 6,148,677 to Evangelista. The flow rig simulates the behaviour of the engine and comprises a wind tunnel set up that measures the pressure drop of an airflow as air passes through the stator ring in a controlled experimental environment. A flow rig may be precalibrated so that a known flow area results in a known pressure drop. Measuring the pressure drop across a particular stator ring therefore can be used to calculate the approximate flow area of that stator ring.
Standard flow rigs require significant set up and the time required to run the flow ring is approximately 45–60 minutes. As well, although flow rigs provide reliable results when stator blades are relatively new and regular, once a worn stator is used with rework or adjustments made to the leading or trailing edges, local pressure effects create significant inaccuracies. In cases where the stator ring has been refurbished, subjected to wear and tear, or has been adjusted excessively, conventional gas flow area measurement with comparator methods are unreliable. A stator ring is an extremely expensive component and therefore an accurate reliable means of measuring the flow area is required.
Another conventional method of determining the flow area of the stator ring involves mechanically measuring the dimensions of the throat area. U.S. Pat. No. 4,222,172 to Mason describes a vane area measurement device using a dial gauge mounted in a specialized fixture to measure the dimension of the throat area. Coordinate measuring machines (CCM) can also trace the area mechanically and calculate the enclosed area which represents the vane throat area.
Mechanical measuring devices may be imprecise, slow and relatively expensive. Coordinate measuring machines currently use two measurement methods to determine throat area. One method measures the throat opening width at three sections and measures the height to obtain the throat area. The first method is imprecise where the trailing edge is irregular between the measured sections. The second method traces the throat opening with a probe without breaking contact. The probe traces the opening at a pre-determined axial distance from the airfoil stacking line, the reference axis of the airfoil. The throat area value calculation assumes that the chord length is constant and that there is no profile deviation between the path traced and the actual trailing edge. However, deviations are common in all but new parts since vane adjustments, refurbishment and normal wear cause profile deviation, chord length deviation or both. The distance between the probe tracing path and the trailing edge may be between 0.050–0.100 inches whereas profile deviation may extend to 0.300–0.400 inches from the trailing edge. Therefore the deviations are not entirely missed by the tracing probe, although a degree of error is introduced.
It is an object of the present invention to provide a fast, inexpensive and reliable method of calculating the throat area of a vane ring.
It is a further object of the invention to utilize optical measurement of the vane ring throat area to avoid the innaccuracy of prior art mechanical and airflow measurement methods.
Further objects of the invention will be apparent from review of the disclosure, drawings and description of the invention below.